In U.S. Pat. No. 3,661,423, assigned to the same assignee as the present invention, there is described an in situ oil shale retorting process. An in situ oil shale retort is a cavity formed in essentially undisturbed walls of oil shale and filled with fragmented oil shale. That patent describes one way of forming an in situ oil shale retort, namely by mining a lower portion of the oil shale formation and then explosively expanding the overlying oil shale to form a large cavity and simultaneously fill it with fragmented oil shale. Other techniques may be suitable for forming an in situ oil shale retort. Kerogen in oil shale in the in situ retort is converted to liquids and product gases by advancing a retorting zone through the in situ retort. The retorting zone can be advanced from the top to the bottom of the in situ retort and the liquids and retort off gases produced can be removed through an access tunnel leading to the bottom of the in situ retort.